Capture Device Design for Furnace Emissions

Transcription

1 Capture Device Design for Furnace Emissions P Carney Limited IE RP-0001, Issue: A Customer Project Number: Customer Document Number: Issue date: 15 November 2016

2 Document Sign Off Capture Device Design for Furnace Emissions P Carney Limited Customer Project Number: Customer Document Number: File No: IE CURRENT ISSUE Issue No: A Date: Reason for issue: For Information Sign Off Originator Checker Reviewer Approver Customer Approval (if required) Print Name Pat Swords Peter R O Sullivan Signature Authorised Electronically Pat Swords Date PREVIOUS ISSUES Issue No Date Originator Checker Reviewer Approver Customer Reason for issue 162.TP.09, Issue 7, 31/03/2014

3 Contents 1 Introduction 4 2 Summary of Conclusions and Recommendations 4 3 Data for Calculation 5 4 Calculation According to VDI 2262 Part Methodology Calculations Capture Element Design 15 IE RP-0001_A_01.DOCX Page 3 of 17

4 1 Introduction The purpose of this document is to provide a methodology for designing a capture device, which is to be located over the furnace outlet at P Carney Ltd, in Crossakiel, Kells, Co. Meath. The furnace gasses exit through a recuperator and are then caught by a capture device, before being routed to a ceramic flue gas filtration system prior to discharge. The purpose of the capture device is not only to capture the hot flue gases, but also to entrain cooling air, which is essential to reduce the temperature of the flue gases, as quickly as possible, outside of the de novo synthesis range for dioxins. Note: the Best Available Techniques (BAT) Reference Document for the Non-Ferrous Metals Industries, which addresses the secondary aluminium sector, it its Section Techniques to reduce dioxins emissions states: - The greater part of PCDD/F formation occurs through de novo synthesis as the off-gas is being cooled through the temperature window of approximately 400 C to 200 C in the presence of reactive carbon, chlorine, oxygen and a catalytically active metal like copper. The methodology used was the German standard VDI 2262 Part 4:2006 Workplace air - Reduction of exposure to air pollutants - Capture of air pollutants, which is a detailed guideline for the dimensioning and design for capture of air pollutants generated during production, treatment and transportation of goods and products, which are emitted to air. In addition this was supplemented by the report of the German Statutory Accident Insurers, BGIA Report 5/2005 Lufttechnik in Industriehalle 1, which relates to ventilation technology in industry halls. The scope of both of these and the worked examples they contain, include molten metal processing. Furthermore, German safety legislation is based on compliance with Stand der Technik, which is state of the art technology. The standards and guidelines of the German Association of Engineers (VDI) and the German Statutory Accident Insurers (Berufsgenossenschaft) are recognising as prescribing Stand der Technik within the German legal structure. They are also more detailed in engineering detail than that to be found in the Health and Safety Authority s equivalent guidance 2. 2 Summary of Conclusions and Recommendations The function of the capture device is to both entrain cooling air and ensure effective capture of the hot flue gases leaving the recupator on the furnace. Currently, there is a hood located right over the discharge position of the recupator and as a result the degree of entrained air is limited and the temperature in the exhaust line at this point is too high, i.e. > 200⁰C. However, it was felt with this approach, i.e. a degree of enclosure of the recupator vent that this would protect the roof above from impingement of hot flue gases and hence a potential fire risk. The calculations and design considerations in the report demonstrate that the most suitable capture element for this application is a nozzle plate, which in essence is a flat plate of a width of at least three and preferentially five duct diameters. This can then be raised above the recupator discharge, such that cooling air is entrained into the duct, but it is also a design, which has a very effective capture efficiency, particularly for thermal discharges, such as in the metal industry, where it is the recommended design. Therefore, provided the exhaust fan is kept running during periods when the furnace is firing, this nozzle plate will provide an effective capture of the recupator discharge plus entrain air to cool that discharge. The degree of entrained air is related to the height of the nozzle plate above the recupator, for which calculations have been completed to bring the temperature in the duct to 200⁰C, although these calculations only provide an approximation. This calculated approximation demonstrates that distance above the recupator should not be less than 0.7 m and hence it is recommended that it be closer to the 1.4 m of height actually available in the furnace area. Indeed, it is only by actual plant trials that this temperature balancing can be confirmed n_lev_guidance.pdf IE RP-0001_A_01.DOCX Page 4 of 17

5 3 Data for Calculation The furnace operates at temperatures up to 1,000⁰C and the heat input is supplied by three burners: - Burner type and size: AFMG 02 - Combustion air flow: 1,000 Nm 3 /h per burner See information below from Fives North American, the burner manufacturer 3 Therefore, the exit flue gas flow from the furnace is assumed to be 3,000 Nm 3 /h. The recuperator is to be upgraded and its function is to cool the flue gas leaving the furnace to a design temperature of 500⁰C, and by means of this heat transfer preheating the inlet air to the burners to 350⁰C. Currently the recuperator is not achieving the necessary preheat and air ballast is being added in to the recuperator to reduce its temperature to the design of 500⁰C, otherwise a temperature of 680⁰C would be achieved. The specific heat capacity of air is readily available in the literature 4 and a figure of 1.1 kj/kg K is representative of the range 500⁰C to 680⁰C. The mass flow of air from the burners, for an air density of 1.2 kg/nm 3 at 20⁰C 5 is: - 3,000 x 1.2 = 3,600 kg/h The total amount of air from the recuperator comes from the following heat equation where temperature in Kelvin (K) is equal to ⁰C plus 273 and the specific heat capacity of ambient air is ( (? + 3,600) x 1.1 x 773) = (3,600 x 1.1 x 953) + (? x x 293) = Q - Solving this equation to determine?, the amount of air ballast to the recuperator gives: -? = 1,282 kg/h - Total air from recuperator at 500⁰C = 4,882 kg/h At 500⁰C the density of air is kg/m 3 from literature 6. Therefore volumetric flow of air from recuperator at this temperature is: - 4,882 / = 10,706 m 3 /h The current design has an 800 mm diameter outlet on the recuperator, which will be reduced to 700 mm on the replacement unit. The cross sectional area of the recuperator outlet is: x / 4 = m 2 The velocity exiting this duct is therefore: - 10,706 / (0.385 x 3,600) = 7.7 m/s Such as: IE RP-0001_A_01.DOCX Page 5 of 17

6 If we consider that sufficient entrained air should be drawn into the capture device, such that by the time the flue gas enters the duct it is reduced in temperature to 200⁰C. For the range 200⁰ to 500⁰C the specific heat capacity of air is about 1.06 kj/kg K. In a similar manner to previous for the ballast air the following can be derived: ( (? 1 + 4,882) x 1.06 x 473) = (4,882 x 1.06 x 753) + (? 1 x x 293) = Q Solving this equation to determine? 1, the amount of entrained air to the capture device gives.? 1 = 7,507 kg/h - Total air from capture device at 200⁰C = 12,389 kg/h - For air at a density of kg/m 3 at 200⁰C, this equates to 16,607 m 3 /h - For air at a density of 1.2 kg/nm 3 at 20⁰C, this equates to 10,324 Nm 3 /h. Note: This is generally in line with what Glosfume measured in April 206 given that the existing recuperator is not performing to its design intent. IE RP-0001_A_01.DOCX Page 6 of 17

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8 4 Calculation According to VDI 2262 Part Methodology VDI 2263 Part 3 is clear in that the dimensioning methods for determining the design of an extraction system are based either on numerical methods or experimental methods. Furthermore, that the efficiency of a capture device can be reduced by cross flows (due to, for example, open doors or gates or leaks in the building envelope) or local air movements (due to, for example, cooling fans or conveyor movements). Therefore, while a numerical analysis using the available calculation methods can provide an initial starting point for a design; this may well have to be refined / adjusted by actual plant trials. The following sections from VDI 2262 Part 4 describe the methodology: IE RP-0001_A_01.DOCX Page 8 of 17

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14 4.2 Calculations From Equation (25): - σ s = 1.4 for Figure 25 for nozzle plate or overhood. - W EM = 7.7 m/s the exit velocity from the recuperator - W quer is assumed to be 3 m/s equivalent to a light breeze of 11 km/h - Z ER /D = 1 / 0.7 = 1.4 based on the diameter of the recuperator emission point and an initial estimate of the height of the capture element of 1 m. - From Figure 26, σ quer = 1.7 and note is not that sensitive for Z ER around the range of 1 m. Therefore using Equation 25 and given that V ER is the desired volume in the extract duct of 16,607 m 3 /h equivalent to 4.61 m 3 /s: = V S (Z ER ) x 1.4 x V S (Z ER ) = 1.93 m 3 /s Using this value then as input for V S ( X ) for equation (20) and noting that the hydraulic diameter d hyd for a circular duct is the same as the actual diameter (0.7 for recuperator) and V 0 is the emission flowrate from the recuperator, which is 10,706 m 3 /h equivalent to 2.97 m 3 /s. Assume for m an intermediate value of 0.45 then: X = [(1.93 / 2.97) / (2 x 0.45)] x 0.7 = 0.5 m Reiterating the calculations with a height of the capture element of 0.6 and recalculating from Figure 26 gives: X = 0.66 m Therefore, an initial estimate of the height of the capture element is about 0.7 m. IE RP-0001_A_01.DOCX Page 14 of 17

15 4.3 Capture Element Design If we consider Section of the BGIA Report 5/2005 then this states: - Figure 9 shows the flow relationships of a nozzle plate. The advantage lies in the radial to centre directed inlet flow. It constricts the convection flow and as a consequence is more stable with respect to cross flows. Furthermore, a velocity filed forms in front of the nozzle plate, which reaches further into the room and thereby increases the depth of capture. Figure 9 from BGIA Report 5/2005 showing the flow conditions with a nozzle plate Therefore, if it is considered that cross flows, such as draughts are a problem, then the nozzle plate design may offer advantages over that of a hood. Note the cover page of a further report from the German Statutory Accident Insurers, BGI 5121 work place ventilation, decision aids for the operational practice 7, shows the following and notes that nozzle plate capture elements find good applicability with thermal flows: Cover page of BGI 5151 showing a nozzle plate operating on a thermal process A typical velocity range for fumes in a duct is 10 m/s, see for instance see Chapter 7 of the Local Exhaust Ventilation guidance from the Irish Health and Safety Authority previously referenced. 7 IE RP-0001_A_01.DOCX Page 15 of 17

16 Given a capture flow at 200⁰C of 16,607 m 3 /h equivalent to 4.61 m 3 /s, then the equivalent duct area is: / 10 = 0.46 m 2 which is equivalent to a 0.75 m duct If we consider Figure 10 of BGIA Report 5/2005 it shows the following performance factors for different nozzle plate and capture hood designs: Figure 10 of BGIA Report 5/2005 relating to performance factors for capture elements Therefore, the width of the nozzle plate should be at least three times the collection duct diameter. The same BGIA Report 5/2005 then goes on to shown in its Figures 11 and 12 the flow conditions at a melt oven. This melt oven was initially fitted with a hood design, which because of its reduced capture depth with small cross flows was subject to pollutants dispersing and not being caught. Once fitted with a nozzle plate, which also with a stronger thermal flow, captured the pollutants safely with the same extraction flow. Figure 11 from BGIA Report 5/2005 flow characteristics of a melt oven with a convention capture hood IE RP-0001_A_01.DOCX Page 16 of 17

17 Figure 12 from BGIA Report 5/2005 flow characteristics of a melt oven with a nozzle plate IE RP-0001_A_01.DOCX Page 17 of 17